Spinal surgeries such as laminectomies, discectomies, fusions and the like are needed by a great many patients. The fact that the spine is a complex construction of bone, cartilage and nerves surrounded by relatively strong muscles makes spinal surgery difficult to perform and requires a high degree of skill on the part of the surgeon if successful results are to be obtained. Initially, all such spinal surgeries were carried out by what is referred to as “open” procedures wherein the spinal structures being operated upon were exposed via a relatively large skin incision that narrows down in conical fashion to the deep, bony operative target, cutting and destroying intervening soft tissue structures. Formation of the large open incision involved severing and separating a large number of tendons, ligaments, and muscle fibers, and this tissue trauma has been found to cause the patient pain, prolonged hospital stays, prolonged recovery and permanent low back weakness. In addition, many patients were dissatisfied with the scarring resultant from large-scale open procedures. Open procedures and apparatus for their implementation are disclosed in the prior art; see U.S. Pat. Nos. 5,052,373 and 5,363,841.
In an attempt to minimize problems associated with large-scale open spinal surgeries, the prior art developed a number of minimally invasive techniques. These techniques are often referred to as “percutaneous” and are typically implemented through the use of endoscopic devices. Such percutaneous techniques involve minimal (“puncture”) incisions and are less traumatic to the patient. However, endoscopic visualization techniques are limiting insofar as the image provided thereby is a two-dimensional image with compromised resolution, for example an image displayed on a video monitor or visualized through a fiber optic viewing device. Furthermore, many such techniques require that the operation be carried out in a surgical field that is filled with a liquid such as a saline solution, or with a gas such as carbon dioxide. Also, if the endoscope and the surgical instruments are passed, by a cylindrical retractor, through the same access port, depth perception and the normal, bimanual use of instruments is greatly hindered. As a consequence, prior art percutaneous techniques are of limited utility. Such techniques are shown in U.S. Pat. Nos. 5,792,044 and 6,206,826.
As a result of the shortcomings of prior art open and percutaneous surgical techniques, spinal surgeons have sought alternative methods whereby spinal surgery may be carried out with minimal tissue invasion, but with maximized visualization of, and access to, the surgical field. Toward that end, direct visualization techniques have been developed wherein a relatively minor incision is formed in the patient's skin, and underlying tissues are displaced through the use of a dilator device, which may comprise a series of dilator mandrels having progressively greater diameters, or through the use of an expansible cannula. The dilator device displaces muscle tissue with minimal tearing or cutting, in a manner analogous to that of a blunt needle being forced through a woven cloth. Once an appropriately sized channel is dilated through the tissue, a working cannula is disposed in the dilated channel, and surgery takes place through the cannula. While this technique does allow for direct visualization and an open surgical field, the dilator devices and cannulae of the prior art provide a cylindrical working passageway through the tissue. This passage has a fairly high aspect ratio insofar as the diameter of the passage is relatively small compared to the passage length. Consequently, the surgeon has a difficult time manipulating instruments through the long narrow channel; furthermore, the geometry of the channel impedes binocular vision of the surgical field. As a consequence, it is often necessary to reposition the cannula during surgery to provide better visualization and/or access. Such repositioning is time consuming, and can tear muscle tissue or cause other undesired surgical trauma. Furthermore, the cylindrical cannula still limits the surgeon's vision and restricts the use of instruments, since access is still provided through a cylindrical channel.
In partial response to the shortcomings of the aforementioned minimally invasive, cannula based techniques, the prior art has developed a transparent walled cannula device which, following tissue dilation, is disposed in the dilated passage. The transparent walls of the cannula enhance visualization of the surgical site. However, the passage defined through the tissue is still cylindrical, and problems of access remain. Such apparatus, and techniques for its use are disclosed in a publication entitled Dilation Discectomy: A System For The Surgeon, Abram, Leon J. M.D., paper published at the 2001 International Intradiscal Therapy Society (IITS) Meeting, and republished July 2001 by Spinal Concepts Inc. as document number 1999-0006-MKC Rev. A per DCR #1327.
In view of the foregoing, it will be appreciated that there is a need for methods and apparatus whereby a surgeon may carry out spinal surgery under direct visualization with minimal patient trauma. Such techniques should provide a surgeon with a good view of, and access to, the surgical site. Furthermore, it is desirable that such techniques and apparatus be simple and reliable. As will be explained in detail hereinbelow, the present invention provides surgical instruments and techniques which fulfill these requirements. These and other advantages of the invention will be apparent from the drawings, discussion and description which follow.